1. Field of the Invention
The present invention relates to a wide-angle lens, an imaging optical apparatus, and a digital equipment, and in particular to, for example, a wide-angle lens that is used for an interchangeable lens for a single-lens reflex camera and a digital camera and forms an optical image of an object on a silver-halide film or an imaging device (for example, a solid-state image sensor such as a CCD (Charge coupled Device) image sensor, and a CMOS (Complementary Metal-Oxide Semiconductor) image sensor), an imaging optical apparatus that outputs the image of the object captured by the wide-angle lens and the imaging device as an electrical signal, and a digital equipment that is equipped with the imaging optical apparatus having an image input function such as a digital camera.
2. Related Background Art
There have been proposed large aperture (an f-number about 1.4), wide-angle lenses suitable for a camera having a long back focal length (such as a single-lens reflex camera) in such as Japanese Patent Application Laid-Open Nos. 11-30743, 2009-20341, and 2009-58652. The wide-angle lenses proposed in Japanese Patent Application Laid-Open Nos. 11-30743, and 2009-20341 have a negative-positive-positive lens group configuration. The wide-angle lens proposed in Japanese Patent Application Laid-Open No. 2009-58625 has a negative-positive lens group configuration. In a wide-angle lens having a long back focal length, a retrofocus lens type having a preceding negative lens group is common. However, since a retrofocus lens type tends to have unsymmetrical lens power distribution with respect to an aperture stop, variation in optical performance upon focusing becomes large. In order to suppress variation in optical performance upon focusing, a floating method is adopted upon focusing in wide-angle lenses disclosed in Japanese Patent Application Laid-Open Nos. 11-30743, and 2009-20341, and a rear-focusing method is adopted in a wide-angle lens disclosed in Japanese Patent Application Laid-Open No. 2009-58652.
When a floating method as shown in wide-angle lenses disclosed in Japanese Patent Application Laid-Open Nos. 11-30743, and 2009-20341 is adopted, there are fears that the lens barrel construction becomes complicated, and deterioration in optical performance is generated by decentering error upon manufacturing. On the other hand, in a wide-angle lens disclosed in Japanese Patent Application Laid-Open No. 2009-58652, although an f-number of 1.4 is accomplished by a rear-focusing method, improvement of variation in optical performance upon focusing is not sufficient.
A large aperture wide-angle lens having an angle of view 2ω of about 80 to 90 degrees with an f-number of about 1.4 has a problem about correction of aberrations. In other words, when a wide angle of view with a large aperture of an f-number smaller than 1.9 is to be accomplished, correction of sagittal coma generated by a lens surface having strong curvature becomes difficult. When sagittal coma still exists, since MTF (Modulation Transfer Function) in a low spatial frequency range becomes worse, improvement of sagittal coma is expected to a wide-angle lens having an f-number of less than 1.9.